In the area of semiconductor device fabrication, the MOS transistor is a basic building block, wherein the transistor can be controlled to operate either in a digital or analog manner. In the fabrication of MOS transistors, source and drain regions are doped opposite that of a body region or well region in a semiconductor substrate. For example, as illustrated in prior art FIG. 1, source/drain regions 12 are formed in a semiconductor body 14 of a MOS transistor, wherein the source/drain regions 12 are an n-type material and the body region 14 is a p-type material (an NMOS transistor). A gate structure 16, for example, a polysilicon gate electrode 18 overlying a gate dielectric 20, overlies a channel region 22 of the semiconductor body. Sidewall spacers 24 reside on lateral edges of the gate structure 16 to facilitate the spacing of extension regions 26 associated with the source/drains 12. Based on the gate structure 16, a distance between the source/drain regions 12 is defined, which is often referred to as a channel length “L”, while a depth of the transistor, or extent in which the transistor extends transverse to the channel, is often referred to as a width “W” of the device. The width-to-length ratio (W/L) is a factor that influences the drive current of the device, as well as other device performance characteristics.
As transistor devices are scaled down to improve device density, both the width “W” and the channel length “L” dimensions are reduced, giving rise to various fabrication and device performance issues. One problem associated with a reduction in the transistor width “W” is experienced when shallow trench isolation (STI) is employed for device isolation, and that problem is sometimes referred to as the inverse narrow width effect (INWE). As the transistor width is reduced the transistor drive current per unit width changes due to the edge effects that now play an appreciable role in transistor behavior. The gate dielectric thickness, its dielectric constant, and the channel orientation are different at the edges than at the planar center of the channel. The dopant concentration at the edges is different at than the center due to dopant segregation and STI stress induced diffusion at the interface. There is also the impact of STI and liner stress on the mobility near the edge of the channels. The STI edge may not be completely planar and may have a gate wrap around (more gate control) or less gate control depending upon the step height (difference between the top of the oxide over field and the top of the active regions). All these factors alter (raise or lower) threshold voltages of the narrow width device resulting in either reduction or increase in drive current per unit width. When the threshold voltage increases for narrow width devices and the drive current per unit width is reduced it results in weaker SRAM transistors which result in slower memory for example as well as functional problems for given SRAM designs. In such cases there is a need to improve the narrow width effects by mitigating the narrow width effects.
Referring to prior art FIG. 2, a portion of a partially fabricated semiconductor device is illustrated, wherein a plurality of isolation structures 30, or STI trenches, are formed in the semiconductor body 14, thereby separating the body into isolation regions 32 and active areas 34, respectively. Subsequently, transistor devices such as MOS transistors are formed in the active areas 34, wherein a width dimension “W” of the MOS transistors extends between the isolation structures 30 as illustrated. As MOS transistor scaling continues, the distance “W” between the isolation structures decreases.
As illustrated in prior art FIG. 3, after source/drain regions 12 and gate structures 16 are formed in the active areas, defined portions 40 of the active regions near the STI trenches 30 can suffer negative effects, such as undesired dopant diffusion and/or other INWE, due to their proximity to the STI trenches. For relatively large width devices, these negative effects are of minimal consequence. However, at smaller/narrower widths, these negative effects can become problematic.
For example, as illustrated in prior art FIGS. 4 and 5, a plan view of two portions of a MOS transistor are provided, wherein the two devices have differing transistors widths. For example, in prior art FIG. 4, an active area 50 is defined between two laterally extending STI isolation regions 52. A conductive gate electrode 54 extends vertically across the active area between the two isolation regions 52, thereby defining a channel region 56 thereunder in the active area. Due to the INWE, a region 58 exists under the channel near the STI that contributes to altering (reducing or increasing) threshold voltage. For a device width W1, the net impact of the region 58 due to the INWE is relatively insignificant, however, as illustrated in prior art FIG. 5, for smaller transistor widths W2, the INWE will have a substantially greater impact on the resulting device performance.
Therefore there remains a need in the art for improved STI processes and techniques that reduce or alter the impact of the INWE in order to reduce or mitigate the device performance problems associated therewith.